Both the mechanical and electrical properties of many superconducting materials make it advantageous to combine the superconductor with a support material, such as copper. In this way, mechanical support is provided for the superconducting material that may be inherently structurally weak. And should a superconductor revert to normal conduction for any of the well-known reasons, whereby the electrical resistance of the previously superconducting element suddenly increases drastically, the current may be by-passed, and heat absorbed by the copper.
Therefore, in the prior art it is known to produce superconducting rods that comprise a core including superconducting material, in monofilament or multifilament constructions, enclosed in a sheath of highly conductive material, such as copper, by extrusion of a billet through a die. Copper has good extrusion qualities as well as high electrical and thermal conductivity and good heat capacity.
As a result of extruding a billet of superconducting material that has been surrounded with a highly conductive sheath and end caps, the central superconducting material becomes fully bonded to the external copper at the atomic level to an extent that the two components cannot be pulled apart. Thus, the extruded product provides an advantageous solution to the problems associated with superconducting materials, as stated.
However, extrusion processes to produce superconducting rods present difficulties in that direct, indirect or hydrostatic extrusion of assembled billets, that is, monofilament composites or multifilament composites, results in yields of steady state material that are less than 90%, when using conventional prior art technology. The term "steady state material" refers to extruded rods that have, along their length, a generally uniform cross section within prescribed limits of proportionality between the superconducting core and the conventionally conducting sheath.
In the prior art, end effects of extrusion render as scrap both the beginning and the tail end of the extruded rod. The ends of the rod do not have the required uniformity of physical characteristics. In particular, at the tail end of the billet, so-called "tubing" occurs following lengths of steady state, acceptable material. In "tubing", copper from the original sheath or matrix material is drawn into the center of the superconducting alloy core, or a void exists, and usually more than 6-8% of the billet length must be cropped off the tail end after extrusion before this undesirable end effect disappears and the extruded material has acceptably uniform qualities.
At the leading end of the billet, the extruded rod has what is known as a "dog bone" effect before the physical characteristics become uniformly acceptable. Further, in preparing some billets for extrusion, a diffusion barrier, typically of Nb or Ta, is introduced between the superconducting core and the outer sheath of copper so as to prevent diffusion of copper into the core and vice versa, whereby a reaction between titanium and copper, for example, may be prevented. At the leading end of the extruded rod, the initial lengths may lack the diffusion barrier before there is a transition to material of acceptable cross sectional quality. It is generally necessary that approximately 4-6% of the billet length be cropped off at the leading end until a fully formed diffusion barrier appears in the extruded rod.
Thus, in the prior art, in the order of 10% or more of the entire drawn rod is unusable because of unacceptable cross sectional geometry, that is, improper ratios of copper and core material and/or absence of the diffusion barrier at both ends of the rod. As the materials, especially the superconducting core materials, are quite expensive, such extrusion losses have considerable impact on the cost of the finished superconducting products, and its ultimate applications.
The distortions from the desired physical characteristics are a direct result of differences in flow and tensile properties at the extrusion temperature between the core, for example, a NbTi ingot or multiple of NbTi rods, and the shell, for example, copper. The extent of the distortions at both ends of the extruded rod is also dependent upon parameters such as die angle, percentage of area reduction from the original billet to the finished rod, speed of extrusion, and coefficient of friction.
What is needed is a billet extrusion method that provides higher yields of usable superconducting rod materials.